1. Field of the Invention
The invention relates generally to the field of processing of three dimensional seismic data. More specifically, the invention relates to methods for migrating three dimensional seismic data, wherein seismic data recordings comprise converted compressional to shear waves.
2. Background Art
Three-dimensional (“3D”) reflection seismic data are being acquired on an increasingly routine basis. Because of the numbers of seismic receivers used in a typical 3D seismic data survey, 3D seismic surveys typically include large volumes of seismic data. These large volumes of data offer the potential for very high-resolution 3D images of the subsurface geology and subsequent estimation of the earth's physical properties.
In seismic surveying, energy from all of the seismic energy sources used in the survey propagates from a subsurface “scatter point” to all the seismic receivers used in acquiring seismic data. Consequently, all recorded traces (called “input” traces for purposes of seismic processing) can contain energy from a particular scatter-point. Because the input traces have a finite recording time, the scattered energy is restricted to traces within the prestack migration aperture of the scattering point. The objective of prestack migration is to gather the seismic energy from all the recorded traces within the prestack migration aperture and sum it back to the scatter-point location.
The accuracy of the migration is related to the accuracy of the calculated seismic signal travel times that are used for migrating the data. The key point is to calculate accurate travel times in order to have better migration imaging. In homogeneous media, seismic travel times, as functions of offset (equivalent distance between the source and receiver along the surface) and common imaging point (“CIP”), are determinable by a simple analytical equation, commonly referred to as the double-square root (“DSR”) equation. The DSR equation to compute travel times is fundamental in migration. The DSR equation is exact in the sense that there are no error-terms dependent on dip angle and offset angle.
In homogeneous media, the velocity of seismic energy through the various subsurface strata is assumed to be constant, and straight rays (a “ray” being a calculated or determined travel path of seismic energy from the source to a reflection point and back to a receiver) are used for travel time computations at any selected imaging point. For media that are vertically inhomogeneous (meaning that the velocity changes with respect to depth in the earth), the RMS (root mean square) average velocity may be used to calculate travel times. However, if there are such vertical changes in velocity, there will be ray-bending at the interfaces between earth strata having different velocities (refraction), and if straight ray paths are assumed, the ray-bending will not be accounted for in the travel time calculations. As velocity changes become greater, the quality of the migration diminishes if ray-bending is not taken into account.
Although conventional approximation of the travel times (straight ray approximation) is substantially accurate for small offset-to-depth ratios, with increasing ratios of offset-to-depth, the accuracy of such conventional approximation diminishes, because the amount of ray bending or refraction is related to the incidence angle of the seismic energy. The incidence angle increases with offset to depth ratio in the presence of ray bending. Therefore, in migrating long-offset seismic data, conventional approximations (straight ray) are not suitable, because ray-bending is not taken into account.
Ray-bending can be determined by using standard ray tracing techniques, however, standard ray tracing has to be performed at every output (image) location and is thus computationally expensive to perform.